What do lowercase letter "a"s mean in guitar tabs?
I'm curious that you didn't ask about the "m"s also. They are there for the same reason. Both are indications of which right hand finger the composer or editor suggests you use to play the note.
- p is short for pulgar, Spanish for "thumb"
- i is short for indice - "index finger"
- m is short for medio - "middle finger"
- a is short for anular - "ring finger"